1. Field of Invention
This invention relates to the synthesis of ferromagnetic chromium dioxide and, more particularly, to a synthesis process in which acetic acid is used as a reducing agent.
2. State of the Art
Ferromagnetic chromium dioxide generally is manufactured by a two-stage process.
In the first stage, chromium trioxide (CrO.sub.3) is dissolved in water. Particulate chromic oxide (Cr.sub.2 O.sub.3) is then added to form a viscous paste in which the chromic oxide is partially dissolved and partially suspended. Dopants and crystal modifiers, such as iron and antimony, may be added in this stage to assist in obtaining the desired magnetic properties. The quantity of water selected is generally the minimum needed for the reduction and to limit the mass of paste that must be handled in the second stage.
In the second stage, the viscous paste is transferred to a reaction vessel, such as the annular can disclosed in U.S. Pat. No. 4,054,544, where it is heated under pressure to form acicular crystals of ferromagnetic chromium dioxide. Temperatures in the range of 300.degree. to 400.degree. C., and pressures in the range of 300 to 400 atmospheres are typically employed in the second stage.
Chromic oxide used in the process conventionally has been a fine-grained material obtained from thermal decomposition of ammonium dichromate. Large grained material generally has been avoided since undissolved grains serve as a nucleation site for chromium dioxide during the second stage, in competition with the desired nucleation sites provided by antimony or other seed crystal modifiers. Chromium dioxide formed on large grained chromic oxide has inferior magnetic properties compared to that formed on fine grained material.
The fine grained chromic oxide is very expensive and the ammonium dichromate decomposition process must be carefully controlled to avoid a runaway explosion. Thus, it is desirable to reduce the relative proportion of chromic oxide used in the manufacture of ferromagnetic chromium dioxide. Patent application Ser. No. 002311 describes such a process in which certain organic reducing agents are used to replace some or all of the chromic oxide in a partitioned reduction process. In one embodiment acetic acid is employed, in conjunction with an organic reducing agent having a C6 to C12 backbone, to adjust the relative percentage of the reduction that occurs during the first and second stages of the reduction process.
While excellent results are obtained when using the partitioned reduction process, there is a need to provide cooling during the first stage to prevent the temperature from prematurely rising above stage one conditions.